[time-nuts] Question on crystal jumps

Brooke Clarke brooke at pacific.net
Fri Oct 24 00:58:43 UTC 2008

Hi Jim:

I've made similar clocks using PIC micro controllers where the input is the 10 
MHz from the frequency standard.

There's a subtle problem that has to do with if an interrupt occurs during a 3 
cycle long instruction or a 2 cycle long instruction.  This can cause erratic 
behavior if not properly accounted for.

The 1 PPS from your clock should be spot on all the time, not off by a few 
hundred nano seconds.

Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
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Jim Palfreyman wrote:
> This discussion has come at an interesting time.
> I've hooked up an LPRO-101 from ebay to an AVR micro-controller - basically
> using it to clock the processor directly.
> I've written code so the AVR is a clock and I've been comparing my clock's 1
> PPS to the GPS.
> Over a few minutes it hops back and forward a few hundred nsec. All quite
> acceptable given the clock speed.
> When I come back after a few hours it will suddenly be 10-20 usec slow. I
> did see it jump over a few seconds from 200nsec to 17 usec.
> At first I put this down to interference because it coincide with me
> switching off a light. So I put the whole lot into a metal box, shorter
> wires and tried again.
> It still has jumps but this time it is *gaining*.
> Until this post started I'd assumed it was maybe my code and was about to
> dive in and check it.
> Could these crystal "jumps" account for my issues?
> Regards,
> Jim

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